Autonomous emergency braking in a road vehicle, in particular in a utility vehicle, is understood to be a braking process that is started automatically without the driver's involvement. As a result, accidents, in particular rear end accidents, should either be entirely prevented or their consequences mitigated (CMS Collision Mitigation System). Known systems of this type (DE 3637165 A1, DE 10 2008 034 229 A1) use sensors, for example RADAR or video cameras, to sense the space in front of the vehicle equipped with the system. A hazard potential is determined on the basis of the known speed of the vehicle in question, determined by means of the wheel speeds, and the speed of the vehicle traveling in front and the distance from that vehicle, wherein the distance data are acquired by means of the RADAR sensor. Of course, the hazard is then particularly high if the distance between the two vehicles decreases rapidly and the distance is below various threshold values. Furthermore, it is known also to take into account the weather or ice on the road for the hazard potential (DE 10 2004 022 289 A1).
If the specified hazard potential has exceeded a specific threshold, a warning tone is generated in the vehicle, the warning tone being intended to signal the imminent danger to the driver who is possibly inattentive at that particular time.
If the hazard potential exceeds a further threshold, autonomous partial braking of the vehicle can be initiated. This means that the vehicle's brakes are activated with medium force without the driver's involvement.
If the hazard potential exceeds a maximum level, autonomous full braking can be initiated (DE 10 2008 034 229 A1), in which the vehicle's brakes are activated with maximum force without the driver's involvement. Even if an imminent accident can no longer be prevented by this, the consequences of the accident are nevertheless greatly reduced by the dissipation of the kinetic energy of the vehicle in question.
In such autonomous braking operations the wheels of the vehicle in question may enter a slipping state, in particular at high deceleration values or under unfavorable weather conditions, that is, ice on the road. In this context, an ABS (anti-lock brake system) in the vehicle may be activated and temporarily relieve the brake pressure on individual wheels. At any rate, this state results in a situation in which a precise vehicle speed can no longer be determined by means of the wheel speeds since the wheel speeds no longer correspond to the vehicle speed owing to the slip. However, as a result it is also no longer possible to precisely determine the speed of the front vehicle whose distance is measured by a distance sensor usually a RADAR sensor. The same applies if the wheel speeds are not used to determine the speed of the vehicle in question and the speed of the front vehicle but rather the output speeds of the transmission are used.
As a result of the described issues in determining the vehicle speed, it is possible, under certain circumstances, for incorrect evaluations of the accident hazard to occur, which can lead to incorrect premature ending of the emergency braking intervention if the speed of the vehicle in question is incorrectly measured as being too low.